// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// FieldTrial is a class for handling details of statistical experiments
// performed by actual users in the field (i.e., in a shipped or beta product).
// All code is called exclusively on the UI thread currently.
//
// The simplest example is an experiment to see whether one of two options
// produces "better" results across our user population.  In that scenario, UMA
// data is uploaded to aggregate the test results, and this FieldTrial class
// manages the state of each such experiment (state == which option was
// pseudo-randomly selected).
//
// States are typically generated randomly, either based on a one time
// randomization (which will yield the same results, in terms of selecting
// the client for a field trial or not, for every run of the program on a
// given machine), or by a session randomization (generated each time the
// application starts up, but held constant during the duration of the
// process).

//------------------------------------------------------------------------------
// Example:  Suppose we have an experiment involving memory, such as determining
// the impact of some pruning algorithm.
// We assume that we already have a histogram of memory usage, such as:

//   UMA_HISTOGRAM_COUNTS("Memory.RendererTotal", count);

// Somewhere in main thread initialization code, we'd probably define an
// instance of a FieldTrial, with code such as:

// // FieldTrials are reference counted, and persist automagically until
// // process teardown, courtesy of their automatic registration in
// // FieldTrialList.
// // Note: This field trial will run in Chrome instances compiled through
// //       8 July, 2015, and after that all instances will be in "StandardMem".
// scoped_refptr<base::FieldTrial> trial(
//     base::FieldTrialList::FactoryGetFieldTrial(
//         "MemoryExperiment", 1000, "StandardMem", 2015, 7, 8,
//         base::FieldTrial::ONE_TIME_RANDOMIZED, NULL));
//
// const int high_mem_group =
//     trial->AppendGroup("HighMem", 20);  // 2% in HighMem group.
// const int low_mem_group =
//     trial->AppendGroup("LowMem", 20);   // 2% in LowMem group.
// // Take action depending of which group we randomly land in.
// if (trial->group() == high_mem_group)
//   SetPruningAlgorithm(kType1);  // Sample setting of browser state.
// else if (trial->group() == low_mem_group)
//   SetPruningAlgorithm(kType2);  // Sample alternate setting.

//------------------------------------------------------------------------------

#ifndef BASE_METRICS_FIELD_TRIAL_H_
#define BASE_METRICS_FIELD_TRIAL_H_

#include <stddef.h>
#include <stdint.h>

#include <map>
#include <set>
#include <string>
#include <vector>

#include "base/base_export.h"
#include "base/gtest_prod_util.h"
#include "base/macros.h"
#include "base/memory/ref_counted.h"
#include "base/observer_list_threadsafe.h"
#include "base/strings/string_piece.h"
#include "base/synchronization/lock.h"
#include "base/time/time.h"

namespace base {

class FieldTrialList;

class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
public:
    typedef int Probability; // Probability type for being selected in a trial.

    // Specifies the persistence of the field trial group choice.
    enum RandomizationType {
        // One time randomized trials will persist the group choice between
        // restarts, which is recommended for most trials, especially those that
        // change user visible behavior.
        ONE_TIME_RANDOMIZED,
        // Session randomized trials will roll the dice to select a group on every
        // process restart.
        SESSION_RANDOMIZED,
    };

    // EntropyProvider is an interface for providing entropy for one-time
    // randomized (persistent) field trials.
    class BASE_EXPORT EntropyProvider {
    public:
        virtual ~EntropyProvider();

        // Returns a double in the range of [0, 1) to be used for the dice roll for
        // the specified field trial. If |randomization_seed| is not 0, it will be
        // used in preference to |trial_name| for generating the entropy by entropy
        // providers that support it. A given instance should always return the same
        // value given the same input |trial_name| and |randomization_seed| values.
        virtual double GetEntropyForTrial(const std::string& trial_name,
            uint32_t randomization_seed) const = 0;
    };

    // A pair representing a Field Trial and its selected group.
    struct ActiveGroup {
        std::string trial_name;
        std::string group_name;
    };

    // A triplet representing a FieldTrial, its selected group and whether it's
    // active.
    struct BASE_EXPORT State {
        StringPiece trial_name;
        StringPiece group_name;
        bool activated;

        State();
        State(const State& other);
        ~State();
    };

    typedef std::vector<ActiveGroup> ActiveGroups;

    // A return value to indicate that a given instance has not yet had a group
    // assignment (and hence is not yet participating in the trial).
    static const int kNotFinalized;

    // Disables this trial, meaning it always determines the default group
    // has been selected. May be called immediately after construction, or
    // at any time after initialization (should not be interleaved with
    // AppendGroup calls). Once disabled, there is no way to re-enable a
    // trial.
    // TODO(mad): http://code.google.com/p/chromium/issues/detail?id=121446
    // This doesn't properly reset to Default when a group was forced.
    void Disable();

    // Establish the name and probability of the next group in this trial.
    // Sometimes, based on construction randomization, this call may cause the
    // provided group to be *THE* group selected for use in this instance.
    // The return value is the group number of the new group.
    int AppendGroup(const std::string& name, Probability group_probability);

    // Return the name of the FieldTrial (excluding the group name).
    const std::string& trial_name() const { return trial_name_; }

    // Return the randomly selected group number that was assigned, and notify
    // any/all observers that this finalized group number has presumably been used
    // (queried), and will never change. Note that this will force an instance to
    // participate, and make it illegal to attempt to probabilistically add any
    // other groups to the trial.
    int group();

    // If the group's name is empty, a string version containing the group number
    // is used as the group name. This causes a winner to be chosen if none was.
    const std::string& group_name();

    // Finalizes the group choice and returns the chosen group, but does not mark
    // the trial as active - so its state will not be reported until group_name()
    // or similar is called.
    const std::string& GetGroupNameWithoutActivation();

    // Set the field trial as forced, meaning that it was setup earlier than
    // the hard coded registration of the field trial to override it.
    // This allows the code that was hard coded to register the field trial to
    // still succeed even though the field trial has already been registered.
    // This must be called after appending all the groups, since we will make
    // the group choice here. Note that this is a NOOP for already forced trials.
    // And, as the rest of the FieldTrial code, this is not thread safe and must
    // be done from the UI thread.
    void SetForced();

    // Enable benchmarking sets field trials to a common setting.
    static void EnableBenchmarking();

    // Creates a FieldTrial object with the specified parameters, to be used for
    // simulation of group assignment without actually affecting global field
    // trial state in the running process. Group assignment will be done based on
    // |entropy_value|, which must have a range of [0, 1).
    //
    // Note: Using this function will not register the field trial globally in the
    // running process - for that, use FieldTrialList::FactoryGetFieldTrial().
    //
    // The ownership of the returned FieldTrial is transfered to the caller which
    // is responsible for deref'ing it (e.g. by using scoped_refptr<FieldTrial>).
    static FieldTrial* CreateSimulatedFieldTrial(
        const std::string& trial_name,
        Probability total_probability,
        const std::string& default_group_name,
        double entropy_value);

private:
    // Allow tests to access our innards for testing purposes.
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, Registration);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, AbsoluteProbabilities);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, RemainingProbability);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FiftyFiftyProbability);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, MiddleProbabilities);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, OneWinner);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DisableProbability);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, ActiveGroups);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, AllGroups);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, ActiveGroupsNotFinalized);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, Save);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SaveAll);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DuplicateRestore);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedTurnFeatureOff);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedTurnFeatureOn);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_Default);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_NonDefault);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FloatBoundariesGiveEqualGroupSizes);
    FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DoesNotSurpassTotalProbability);

    friend class base::FieldTrialList;

    friend class RefCounted<FieldTrial>;

    // This is the group number of the 'default' group when a choice wasn't forced
    // by a call to FieldTrialList::CreateFieldTrial. It is kept private so that
    // consumers don't use it by mistake in cases where the group was forced.
    static const int kDefaultGroupNumber;

    // Creates a field trial with the specified parameters. Group assignment will
    // be done based on |entropy_value|, which must have a range of [0, 1).
    FieldTrial(const std::string& trial_name,
        Probability total_probability,
        const std::string& default_group_name,
        double entropy_value);
    virtual ~FieldTrial();

    // Return the default group name of the FieldTrial.
    std::string default_group_name() const { return default_group_name_; }

    // Marks this trial as having been registered with the FieldTrialList. Must be
    // called no more than once and before any |group()| calls have occurred.
    void SetTrialRegistered();

    // Sets the chosen group name and number.
    void SetGroupChoice(const std::string& group_name, int number);

    // Ensures that a group is chosen, if it hasn't yet been. The field trial
    // might yet be disabled, so this call will *not* notify observers of the
    // status.
    void FinalizeGroupChoice();

    // Returns the trial name and selected group name for this field trial via
    // the output parameter |active_group|, but only if the group has already
    // been chosen and has been externally observed via |group()| and the trial
    // has not been disabled. In that case, true is returned and |active_group|
    // is filled in; otherwise, the result is false and |active_group| is left
    // untouched.
    bool GetActiveGroup(ActiveGroup* active_group) const;

    // Returns the trial name and selected group name for this field trial via
    // the output parameter |field_trial_state|, but only if the trial has not
    // been disabled. In that case, true is returned and |field_trial_state| is
    // filled in; otherwise, the result is false and |field_trial_state| is left
    // untouched.
    bool GetState(State* field_trial_state);

    // Returns the group_name. A winner need not have been chosen.
    std::string group_name_internal() const { return group_name_; }

    // The name of the field trial, as can be found via the FieldTrialList.
    const std::string trial_name_;

    // The maximum sum of all probabilities supplied, which corresponds to 100%.
    // This is the scaling factor used to adjust supplied probabilities.
    const Probability divisor_;

    // The name of the default group.
    const std::string default_group_name_;

    // The randomly selected probability that is used to select a group (or have
    // the instance not participate).  It is the product of divisor_ and a random
    // number between [0, 1).
    Probability random_;

    // Sum of the probabilities of all appended groups.
    Probability accumulated_group_probability_;

    // The number that will be returned by the next AppendGroup() call.
    int next_group_number_;

    // The pseudo-randomly assigned group number.
    // This is kNotFinalized if no group has been assigned.
    int group_;

    // A textual name for the randomly selected group. Valid after |group()|
    // has been called.
    std::string group_name_;

    // When enable_field_trial_ is false, field trial reverts to the 'default'
    // group.
    bool enable_field_trial_;

    // When forced_ is true, we return the chosen group from AppendGroup when
    // appropriate.
    bool forced_;

    // Specifies whether the group choice has been reported to observers.
    bool group_reported_;

    // Whether this trial is registered with the global FieldTrialList and thus
    // should notify it when its group is queried.
    bool trial_registered_;

    // When benchmarking is enabled, field trials all revert to the 'default'
    // group.
    static bool enable_benchmarking_;

    DISALLOW_COPY_AND_ASSIGN(FieldTrial);
};

//------------------------------------------------------------------------------
// Class with a list of all active field trials.  A trial is active if it has
// been registered, which includes evaluating its state based on its probaility.
// Only one instance of this class exists.
class BASE_EXPORT FieldTrialList {
public:
    // Year that is guaranteed to not be expired when instantiating a field trial
    // via |FactoryGetFieldTrial()|.  Set to two years from the build date.
    static int kNoExpirationYear;

    // Observer is notified when a FieldTrial's group is selected.
    class BASE_EXPORT Observer {
    public:
        // Notify observers when FieldTrials's group is selected.
        virtual void OnFieldTrialGroupFinalized(const std::string& trial_name,
            const std::string& group_name)
            = 0;

    protected:
        virtual ~Observer();
    };

    // This singleton holds the global list of registered FieldTrials.
    //
    // To support one-time randomized field trials, specify a non-NULL
    // |entropy_provider| which should be a source of uniformly distributed
    // entropy values. Takes ownership of |entropy_provider|. If one time
    // randomization is not desired, pass in NULL for |entropy_provider|.
    explicit FieldTrialList(const FieldTrial::EntropyProvider* entropy_provider);

    // Destructor Release()'s references to all registered FieldTrial instances.
    ~FieldTrialList();

    // Get a FieldTrial instance from the factory.
    //
    // |name| is used to register the instance with the FieldTrialList class,
    // and can be used to find the trial (only one trial can be present for each
    // name). |default_group_name| is the name of the default group which will
    // be chosen if none of the subsequent appended groups get to be chosen.
    // |default_group_number| can receive the group number of the default group as
    // AppendGroup returns the number of the subsequence groups. |trial_name| and
    // |default_group_name| may not be empty but |default_group_number| can be
    // NULL if the value is not needed.
    //
    // Group probabilities that are later supplied must sum to less than or equal
    // to the |total_probability|. Arguments |year|, |month| and |day_of_month|
    // specify the expiration time. If the build time is after the expiration time
    // then the field trial reverts to the 'default' group.
    //
    // Use this static method to get a startup-randomized FieldTrial or a
    // previously created forced FieldTrial.
    static FieldTrial* FactoryGetFieldTrial(
        const std::string& trial_name,
        FieldTrial::Probability total_probability,
        const std::string& default_group_name,
        const int year,
        const int month,
        const int day_of_month,
        FieldTrial::RandomizationType randomization_type,
        int* default_group_number);

    // Same as FactoryGetFieldTrial(), but allows specifying a custom seed to be
    // used on one-time randomized field trials (instead of a hash of the trial
    // name, which is used otherwise or if |randomization_seed| has value 0). The
    // |randomization_seed| value (other than 0) should never be the same for two
    // trials, else this would result in correlated group assignments.  Note:
    // Using a custom randomization seed is only supported by the
    // PermutedEntropyProvider (which is used when UMA is not enabled). If
    // |override_entropy_provider| is not null, then it will be used for
    // randomization instead of the provider given when the FieldTrialList was
    // instanciated.
    static FieldTrial* FactoryGetFieldTrialWithRandomizationSeed(
        const std::string& trial_name,
        FieldTrial::Probability total_probability,
        const std::string& default_group_name,
        const int year,
        const int month,
        const int day_of_month,
        FieldTrial::RandomizationType randomization_type,
        uint32_t randomization_seed,
        int* default_group_number,
        const FieldTrial::EntropyProvider* override_entropy_provider);

    // The Find() method can be used to test to see if a named trial was already
    // registered, or to retrieve a pointer to it from the global map.
    static FieldTrial* Find(const std::string& trial_name);

    // Returns the group number chosen for the named trial, or
    // FieldTrial::kNotFinalized if the trial does not exist.
    static int FindValue(const std::string& trial_name);

    // Returns the group name chosen for the named trial, or the empty string if
    // the trial does not exist. The first call of this function on a given field
    // trial will mark it as active, so that its state will be reported with usage
    // metrics, crashes, etc.
    static std::string FindFullName(const std::string& trial_name);

    // Returns true if the named trial has been registered.
    static bool TrialExists(const std::string& trial_name);

    // Returns true if the named trial exists and has been activated.
    static bool IsTrialActive(const std::string& trial_name);

    // Creates a persistent representation of active FieldTrial instances for
    // resurrection in another process. This allows randomization to be done in
    // one process, and secondary processes can be synchronized on the result.
    // The resulting string contains the name and group name pairs of all
    // registered FieldTrials for which the group has been chosen and externally
    // observed (via |group()|) and which have not been disabled, with "/" used
    // to separate all names and to terminate the string. This string is parsed
    // by |CreateTrialsFromString()|.
    static void StatesToString(std::string* output);

    // Creates a persistent representation of all FieldTrial instances for
    // resurrection in another process. This allows randomization to be done in
    // one process, and secondary processes can be synchronized on the result.
    // The resulting string contains the name and group name pairs of all
    // registered FieldTrials which have not been disabled, with "/" used
    // to separate all names and to terminate the string. All activated trials
    // have their name prefixed with "*". This string is parsed by
    // |CreateTrialsFromString()|.
    static void AllStatesToString(std::string* output);

    // Fills in the supplied vector |active_groups| (which must be empty when
    // called) with a snapshot of all registered FieldTrials for which the group
    // has been chosen and externally observed (via |group()|) and which have
    // not been disabled.
    static void GetActiveFieldTrialGroups(
        FieldTrial::ActiveGroups* active_groups);

    // Returns the field trials that are marked active in |trials_string|.
    static void GetActiveFieldTrialGroupsFromString(
        const std::string& trials_string,
        FieldTrial::ActiveGroups* active_groups);

    // Use a state string (re: StatesToString()) to augment the current list of
    // field trials to include the supplied trials, and using a 100% probability
    // for each trial, force them to have the same group string. This is commonly
    // used in a non-browser process, to carry randomly selected state in a
    // browser process into this non-browser process, but could also be invoked
    // through a command line argument to the browser process. Created field
    // trials will be marked "used" for the purposes of active trial reporting
    // if they are prefixed with |kActivationMarker|. Trial names in
    // |ignored_trial_names| are ignored when parsing |trials_string|.
    static bool CreateTrialsFromString(
        const std::string& trials_string,
        const std::set<std::string>& ignored_trial_names);

    // Create a FieldTrial with the given |name| and using 100% probability for
    // the FieldTrial, force FieldTrial to have the same group string as
    // |group_name|. This is commonly used in a non-browser process, to carry
    // randomly selected state in a browser process into this non-browser process.
    // It returns NULL if there is a FieldTrial that is already registered with
    // the same |name| but has different finalized group string (|group_name|).
    static FieldTrial* CreateFieldTrial(const std::string& name,
        const std::string& group_name);

    // Add an observer to be notified when a field trial is irrevocably committed
    // to being part of some specific field_group (and hence the group_name is
    // also finalized for that field_trial).
    static void AddObserver(Observer* observer);

    // Remove an observer.
    static void RemoveObserver(Observer* observer);

    // Notify all observers that a group has been finalized for |field_trial|.
    static void NotifyFieldTrialGroupSelection(FieldTrial* field_trial);

    // Return the number of active field trials.
    static size_t GetFieldTrialCount();

private:
    // A map from FieldTrial names to the actual instances.
    typedef std::map<std::string, FieldTrial*> RegistrationMap;

    // If one-time randomization is enabled, returns a weak pointer to the
    // corresponding EntropyProvider. Otherwise, returns NULL.
    static const FieldTrial::EntropyProvider*
    GetEntropyProviderForOneTimeRandomization();

    // Helper function should be called only while holding lock_.
    FieldTrial* PreLockedFind(const std::string& name);

    // Register() stores a pointer to the given trial in a global map.
    // This method also AddRef's the indicated trial.
    // This should always be called after creating a new FieldTrial instance.
    static void Register(FieldTrial* trial);

    static FieldTrialList* global_; // The singleton of this class.

    // This will tell us if there is an attempt to register a field
    // trial or check if one-time randomization is enabled without
    // creating the FieldTrialList. This is not an error, unless a
    // FieldTrialList is created after that.
    static bool used_without_global_;

    // Lock for access to registered_.
    base::Lock lock_;
    RegistrationMap registered_;

    std::map<std::string, std::string> seen_states_;

    // Entropy provider to be used for one-time randomized field trials. If NULL,
    // one-time randomization is not supported.
    std::unique_ptr<const FieldTrial::EntropyProvider> entropy_provider_;

    // List of observers to be notified when a group is selected for a FieldTrial.
    scoped_refptr<ObserverListThreadSafe<Observer>> observer_list_;

    DISALLOW_COPY_AND_ASSIGN(FieldTrialList);
};

} // namespace base

#endif // BASE_METRICS_FIELD_TRIAL_H_
